The present invention relates to a hand-off method and a mobile station for a spread spectrum cellular mobile communication system.
The cellular mobile communication system is now operational as a digital automobile telephone system. In the digital mobile telephone system, as shown in FIG. 1A, a service area 1 is split into a plurality of radio zones 2a, 2b, 2c, . . . , which are assigned control channels 3a, 3b, 3c, . . . of individual frequencies, and the control channels 3a, 3b, 3c, . . . are transmitted from radio base stations 4a, 4b, 4c, . . . installed in the radio zones 2a, 2b, 2c, . . . , respectively. When a mobile station 5 hands off during conversation, it measures the received levels of the control channels 3a, 3b, 3c, . . . from the radio zones 2a, 2b, 2c, . . . one after another through utilization of idle time slots I of the TDMA scheme which transmits and receives in specified time slots (Tx) and (Rx) as shown in FIG. 1B, then compares the measured received levels and selects the radio zone of the maximum received level as a destination or new radio zone.
On the other hand, in the spread spectrum mobile communication system, a single radio frequency is assigned in common to all radio zones involved and a plurality of spectrum spreading codes are assigned to each radio zone; one possible method for hand-off in this system is to scan the spectrum spreading codes by a correlator of the mobile station, select a destination radio zone by measuring and comparing the received levels every radio zone and set the spectrum spreading codes of the destination radio zone in the correlator. In the spread spectrum mobile communication system, however, it is necessary to prepare a wide-band radio frequency channel so that when traffic of the radio zones is unbalanced, many spectrum spreading codes could be used to accommodate or deal with the traffic imbalance; this inevitably leads to overdesign of low-traffic radio zones.
To avoid this, it is possible to employ a system configuration wherein a plurality of radio channels of different frequencies but of the same bandwidth corresponding to a certain traffic volume are prepared, radio zones of low traffic are assigned only one of the radio channels, radio zones of high traffic are assigned two or more of the radio channels and the radio channel of the same frequency is assigned a different spectrum spreading code at least every group of adjacent radio zones. In this instance, each communication channel is defined by a set (f, c) of the radio frequency f and the spectrum spreading code C. In the following description, the channel of the radio 20 frequency f that is assigned one or more spectrum spreading codes will be referred to as a radio channel. Hence, each radio channel is allowed to contain a plurality of communication channels defined by different spectrum spreading codes.
As indicated by the traffic volume distribution curve 6 in FIG. 2, traffic tends to be high in the central city area but decrease with distance therefrom. In such a situation, the number of radio channels necessary for dealing with traffic is selected large in the central area of the city but smaller with an increase in the distance therefrom as indicated by the channel numbers F1, F2, F3, assigned to the zones 2a, 2b, 2c, . . . in FIG. 2. To hand off during conversation in this system configuration, the mobile station determines its destination radio zone on the basis of the received level of a predetermined communication channel (a control channel or broadcasting channel for zone selection use, for example) from every radio zone of the radio channel F1 of the frequency f common to all radio zones. Accordingly, to select the destination zone during conversation over a radio channel other than the common radio channel F1 in the radio zone assigned the plurality of radio channels F1, F2, . . . , it is necessary to measure the received level in the common channel F1 and this requires interruption of the conversation. To enable measurement of the received level without interrupting the conversation, two receiving systems need to be prepared--this goes against the demand for miniaturization of the mobile station.
A possible solution to the above-noted problem is to employ a TDMA-like radio channel scheme as in the digital automobile telephone system and utilize a TDMA-type idle time slot to detect the destination zone. With this method, however, since signals become burst-like, the electromagnetic compatibility (EMC) of the system must be taken into account, besides guard bits and synchronization words are needed, impairing the efficiency in usage of the channel.